1. Field of the Invention
The present invention relates to an ink, and particularly to an ink used for ink-jet-system printing. The present invention can be applied to apparatuses, for example, office equipment employing an ink jet recording system, such as a printer, a copying machine, a facsimile, etc., which are capable of recording on various recording media such as paper, a cloth, a leather, a nonwoven fabric, OHP paper, etc. The present invention also relates to an ink cartridge, a recording unit, an ink jet recording apparatus and an ink jet recording method.
2. Description of the Related Art
An ink jet printing method has the advantages of low noise and low running cost, and the advantages that high-speed printing can be performed, the apparatus used can be easily decreased in size, and color printing can easily be performed. This printing method is widely used in a printer, a copying machine, etc. in which the ink used is generally selected from the viewpoints of printability such as discharge performance, fixing performance, and the like, blurring and an optical reflection density of a printed image, print quality such as a coloring property, etc.
It is widely known that inks used for ink jet printing are roughly divided into two types including dye-based ink and pigment-based ink. The pigment-based ink has advantages that the water resistance and light fastness are superior to the dye-based ink, and clear characters can be printed.
A pigment contained in the pigment-based ink is stably dispersed in the ink by using a dispersant such as a polymeric dispersant, or the like. More specifically, the polymeric dispersant is adsorbed on pigment particles to overcome the intermolecular force exerting between the pigment particles, which causes agglomeration of the pigment particles, by using the electric replusive force of the polymeric dispersant, thereby stably dispersing the pigment particles in the ink. Therefore, the polymeric dispersant must be added to the ink according to the amount of the pigment.
In printing on plain paper by an ink jet recording method using such ink, an ink solvent such as water or the like permeates into paper, or evaporates into the air to cause agglomeration of the pigment. In this case, as a behavior on the paper, the force of agglomeration increases as the amount of the polymeric dispersant increases. Therefore, the diameter of an ink dot formed on a printing medium is smaller than that of an ink dot formed with the dye-based ink conventionally used for ink jet printing, and the dot shape is close to a distorted shape immediately after collision with the paper. Therefore, in order to obtain ink dots having a diameter necessary for forming an image having a sufficient record density without causing white stripes or the like, the volume of the ink discharged from an ink jet head must be controlled to be as large as possible. However, this causes a delay in ink fixing to the printing medium in combination with a decrease in permeability into the paper due to the strong force of agglomeration of the pigment particles adsorbing the polymeric dispersant, or cause deterioration in abrasion resistance of a recorded image.
In order to increase the dot diameter and improve the fixing performance, a penetrating agent is possibly contained in the ink for improving permeability of the ink into the printing medium. However, this method causes phenomena undesirable for obtaining a high-quality recorded image, such as deterioration in the dot shape, for example, deterioration in the peripheral shape of a dot such as feathering, ink permeation to the back of paper (so-called xe2x80x9cstrike throughxe2x80x9d), etc. Also, a colorant permeates into the printing medium, thereby failing to increase the optical density of a dot even when the dot diameter becomes relatively large.
Furthermore, an ink containing a self-dispersible pigment has been proposed, which can increase the dot diameter due to weak force of agglomeration of the pigment on paper, as compared with ink containing a pigment dispersed by the above-described dispersant. However, the dot diameter cannot be sufficiently increased by this type of ink.
Therefore, an ink and a printing method are in the course of research for further improving properties in order to satisfy various factors which determine the quality of a recorded image, for example, ink fixing performance, the increased diameter of an ink dot, uniformity of density within an ink dot, the high optical density of an ink dot itself, etc., and satisfy stability as an ink, particularly the property that the ink can stably be discharged for ink jet printing.
In the course of research for further improving the quality of an ink jet recorded image, the inventors found that an ink comprising a mixture of a self-dispersible pigment, a pigment which can be dispersed by a polymeric dispersant, and a polymeric dispersant satisfies high levels of various factors required for improving image quality, and has excellent stability.
Namely, even when the total amount of the polymeric dispersant contained in the ink was decreased, the self-dispersible pigment (first pigment) functions as a dispersant for the pigment (second pigment) which can be dispersed in an aqueous medium by the polymeric dispersant to stably maintain the dispersed state of the pigments in the ink until the ink is discharged from a recording head. On the other hand, in printing on paper with the ink, the dot diameter was increased, as compared with an ink containing the second pigment and the polymeric dispersant for dispersing the second pigment, and an ink containing only the first pigment, and the ink uniformly diffused on the paper with a high optical density (O.D.) and a relatively high rate of fixing.
Although the reason why the above phenomenon is observed is not clear, it is supposed to be due to the following mechanism. The second pigment having the polymeric dispersant adsorbed thereon, and the first pigment electrically repel each other in the ink to weaken the force of agglomeration of the pigments, as compared with at least an ink containing only a pigment dispersed by a polymeric dispersant. In printing on paper with the ink, a colorant in the ink less permeates into the paper in the direction of the thickness thereof because the polymeric dispersant is adsorbed on the second pigment. While in the ink containing the second pigment and the polymeric dispersant, the polymer molecules are rapidly entangled with each other or the polymer is cross-linked between the pigment particles in the (transverse) direction of the paper plane accompanying a decrease in the water content due to permeation of the ink solvent into the paper and evaporation of the solvent, thereby strongly agglomerating the pigment. However, in the ink of the present invention, mixing the first pigment prevents or suppresses entanglement or crosslinking of the polymer, and the strong intermolecular force between the ink pigments is relieved by repulsion between the first pigment and the polymeric dispersant. As a result, the ink easily diffuses in the transverse direction of the paper due to the relief, but the effect of the force of agglomeration of the pigments provides diffusion of the ink with regularity. This can possibly ensure the diameter and circularity of a dot with a small amount of discharge, and achieve conformability of a plurality of dots, i.e., a good smoothing property. This phenomenon on paper becomes particularly significant when the Ka value of the ink obtained by the Bristow""s method is less than 1 mlxc2x7mxe2x88x922xc2x7msecxe2x88x92xc2xd, i.e., when permeability for the printing medium is set to be relatively low. Therefore, this phenomenon is advantageous for improving image quality.
The ink of the present invention has good dispersion stability in an ink tank, as described above, and has the great effect on printability that the dot diameter and O.D. can be increased, and abrasion resistance and dot circularity can be improved.
As a result of further research in serious consideration of the excellent ability of the ink, the inventors found an ink composition capable of improving stability of ink jet discharge performance and the quality of a recorded image.
Accordingly, an object of the present invention is to provide an ink capable of stably recording an image having a high area factor and a high image density.
Another object of the present invention is to provide an ink jet recording method capable of stably forming a high-quality image.
A further object of the present invention is to provide an ink jet recording apparatus capable of stably recording a high-quality image, and an ink cartridge and recording unit which can be used in the ink jet recording apparatus.
In order to achieve the objects, in accordance with first embodiment of the present invention, there is provided an ink comprising a first pigment and a second pigment as colorants which are dispersed in an aqueous medium, and a polymeric dispersant, wherein the first pigment is a self-dispersible pigment having at least one anionic group or cationic group bound to the surface thereof directly or through another atomic group, and the second pigment is dispersible in the aqueous medium with a polymeric dispersant, the polymeric dispersant being at least one of a polymeric dispersant having the same polarity as the group bound to the surface of the first pigment and a nonionic polymeric dispersant, and wherein the ink shows one peak in particle diameter""s distribution, the peak positioning at the particle diameter of from 50 to 300 nm.
Namely, as a result of research performed by the inventors in consideration of the above-described characteristics of ink jet recording, with the pigment-based ink comprising the two types of pigments including the self-dispersible pigment (first pigment) and the pigment (second pigment) which can be dispersed in the aqueous medium with the polymeric dispersant, application to ink jet recording could be further optimized by defining the particle diameter distribution of the ink.
In the particle diameter distribution of the pigment-based ink, the particle diameters of the self-dispersible pigment (first pigment) and the pigment (second pigment) which can be dispersed in the aqueous medium by the polymeric dispersant are not simply overlapped with each other. In mixing pigment-based inks having different properties, the particle diameter distribution of the resultant ink is slightly different from the simple sum of the particle diameter distributions of the respective inks. For example, when the particle diameter distributions of two pigments having different particle diameter distributions have peaks close to each other, the particle diameter distribution of the mixed pigment-based ink has one peak. Although the reason why such an ink exhibits the above-described various effects, it is thought that the first and second pigments have particle diameters relatively close to each other to effectively cause the interaction between the first and second pigments. Furthermore, the extreme value of the particle diameters of the ink is controlled in the range of 50 to 300 nm, and more preferably in the range of 50 to 200 nm, to substantially remove fine pigment particles causing deterioration in the optical density, and coarse pigment particles causing deterioration in discharge stability from the ink.
The ink according to the first embodiment of the present invention exhibits good dispersion stability in an ink tank, and the great effect on printability that the dot diameter and the optical density of an image can be improved, and abrasion resistance and dot circularity can be improved.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.